Confined feed (CF) nozzles (also called close-coupled inert gas atomization nozzles), such as annular slit-confined feed and discrete jet-confined feed nozzles (e.g. the high pressure gas atomization HPGA nozzle described in U.S. Pat. No. 5 125 574), are known to be efficient nozzle configurations for producing fine metallic powder by atomizing a suitable melt to form an atomization spray of molten droplets that rapidly solidify to form generally spherical, fine metallic powder. The aforementioned high pressure gas atomization nozzle includes an array of individual gas outlet orifices (jets) located externally of and concentrically to a melt pour tube orifice. Although these nozzles are efficient producers of metallic powder, they require considerable expertise and experience to operate in a commercial production environment in a manner to accommodate various demands of production, such as the need to effect a major shift or change in the mean powder particle size being produced, without having to shut down the powder production operation. Moreover, considerable expertise and experience are required to accommodate major nozzle parameter fluctuations, such as severe transient events including start-up and impending freeze-off of the nozzle, that are experienced without adversely affecting powder production. As a result, the industrial application of such nozzles generally has been limited to a few high technology practitioners, such as producers of nickel base superalloy powder for aircraft gas turbine engines.
The design of most industrial CF atomization nozzles is of the annular slit type and includes a gas outlet in the form of a thin annular slit located externally of and concentric to the melt pour tube orifice. During the typical assembly stage of each atomization run, the annular slit gap, the pour tube position, and the atomization gas pressure, for example, are set according to prescribed parameters based on the experience of the atomization engineer. After the initial settings are made, the engineer does not have the ability to modify the parameters, e.g. slit gap and pour tube position, during the powder production run. Other parameters, e.g. atomization gas pressure, can be varied manually during the run but sometimes not on a sufficiently short time scale to accommodate rapidly occurring/changing transient events or parameters.
Not only do current CF atomization nozzles lack active control devices but also they lack rapid process status sensors. Typically, the atomization process is monitored visually by an operator through a viewport in the side of the spray chamber, primarily to verify that the nozzle is functioning, with, however, the view often being obscured by billowing powders in the spray chamber.
One advancement in feedback sensors for use with powder atomizers; namely, the in-situ particle size analyzer based on scattering of laser light, has been under development for at least six years. This device is intended to determine the as-atomized particle size distribution and provide direct measurement of the effect of a particular set of atomization parameters on particle size. Unfortunately, the data collection and analysis time of this type of laser light scattering device is still minutes and not fractions of a second as required to respond automatically to real atomization parameter fluctuations.
The lack of a fast response time to transient events impedes adequate response and can interfere with or shut off the atomization spray process. Also, the lack of fast response prohibits in-situ tuning of the atomization parameters needed to produce major shifts in mean powder particle size during continuous operation of the atomizing equipment.
It is an object of the present invention to provide an improved gas atomizing apparatus with a control system for providing in-situ control of certain primary nozzle operating parameters to provide enhanced flexibility in the manufacture of powder (e.g. ability to effect a major shift of the desired mean powder particle size) without shut down of the atomization spray process and improved ability to respond to major nozzle parameter fluctuations (e.g. severe transient events such as start-up and impending nozzle freeze-off).